Internal combustion engines, air compressors and other piston equipment typically have compression rings and oil rings. The compression rings are usually circumferentially disposed around the piston end nearest the compression chamber. They form a gas tight seal between the piston and cylinder wall. The oil ring is located below the compression rings and in piston ring grooves. The purpose of the oil ring is to provide oil control and lubrication for the compression ring or rings by efficiently metering a very thin coat of lubricating oil onto the cylinder wall or cylinder liner wall over the entire stroke of the piston.
Piston thrust loads occur at the piston skirt area below the oil ring where an oil film lubricates the two sliding surfaces,, namely the piston skirt and cylinder liner or cylinder. The piston ring assemblies of conventional reciprocating engines requires oil to lubricate piston-skirt/liner which absorbs the piston thrust load. The sliding area is filled with oil and the oil ring controls the amount of oil which reaches the compression ring area. If the oil ring scrapes excessively, it tends to induce scuffing of the cylinder liner. If, on the other hand, the oil ring delivers too much oil, excessive consumption of oil may occur with the result that carbon forms on the top portion of the piston and increasing the chance of blow-by as well as ring sticking. Also, the amount of hydrocarbons in the engine exhaust gas may be higher due to the evaporation of the oil from the liner surface.